Climate Dynamics (1995) 12:21-35 ( limni¢ Dynamies © Springer-Verlag 1995 A new snow parameterization for the M6t o-France climate model Part I: validation in stand-alone experiments H. Douville, J.-F. Royer, J.-F. Mahfouf Mdt6o-France/CNRM, 42 Avenue Coriolis, 31057 Toulouse Cedex, France Received: 7 November 1994 / Accepted: 30 May 1995 Abstract. Both observational studies and numerical ex- periments demonstrate the sensitivity of the atmo- sphere to variations in the extent and mass of snow cover. There is therefore a need for simple but realistic snow parameterizations in forecast and climate models. This study describes a new physically-based snow hy- drology for use in the Mdtdo-France climate model, to- gether with the ISBA land-surface scheme. A re- stricted number of parameters has been added, while preserving a single surface energy budget. The ageing process of the snow pack has been introduced through prognostic equations for snow density and snow albe- do. Snowmelt computation has been modified over partially snow-covered and vegetated areas. The new scheme has been validated against field measurements in stand-alone simulations forced by observed meteor- ological conditions. The results show a strong improve- ment in the model's performance, thereby suggesting that a simple one-layer snow model is able to repro- duce the main physical mechanisms governing the snow pack evolution. Part II of the present study will concern the validation in a 3-D experiment within the Mdt6o-France climate model. 1 Introduction The energy budget of the lower atmosphere is very de- pendent on the physical properties observed at the Earth's surface. From this point of view, the snow cov- er represents a critical component of the climate sys- tem. The high reflectivity of snow can increase the sur- face albedo by as much as 60%. The induced radiative cooling is reinforced by the high thermal emissivity of the snow pack. The turbulent fluxes are reduced, be- cause of the increased static stability in the boundary layer and the decreased roughness length. Fresh snow acts as a thermal insulator due to its low thermal con- Correspondence to: H. Douville ductivity. Snowmelt is a sink for latent heat and re- leases water to the atmosphere and the ground. Of all the surface conditions, snow shows the largest spatial and temporal fluctuations. A single storm can increase the snow cover extent by the order of 1000 km in a few days. On longer time scales, snow depth exhi- bits a strong seasonal cycle, as well as an important in- terannual variability in the middle latitudes. The physi- cal properties of snow are also very variable, due to ageing processes. Snow albedo can vary between 30 and 90%. Snow density increases rapidly with time, evolving from about 80 kg/m 3 to more than 300 kg/m 3. Snow thermal properties are very sensitive to the set- tling of the snow pack. Early studies of the climatic impact of snow have been based on observations and have emphasized the cooling effect of snow on the winter temperatures (De- wey 1977; Namias 1985). Correlations have been found between snow cover and geopotential height anomal- ies, showing that the winter snow pack is determined by the atmospheric dynamics but also feeds back onto the atmosphere (Walsh et al. 1982). More sophisticated statistical approaches have also suggested a high sensi- tivity of the atmosphere to variations in the extent and mass of accumulated snow (Walsh et al. 1985). Other studies have focused on the interannual variability of the Northern Hemisphere snow cover (Iwasaki 1991; Gutzler and Rosen 1992; Groisman et al. 1994). They have shown some relationships between the Eurasian and North American winter snow packs, as well as a long term decrease in the springtime Northern Hemis- phere snow cover extent, which may explain why the recent global warming is more obvious in spring than in any other season. Many recent studies have been with general circula- tion models (GCMs). They have shown the importance of snow for weather forecasts as well as for climate si- mulation (Yeh et al. 1983). The sensitivity of the In- dian monsoon to the Eurasian snow cover has been confirmed by numerical experiments (Barnett et al. 1989; Yasunari et al. 1991). The global warming pre- dicted by climate models in a doubled CO2 atmosphere